Porous graphene based composite membranes for nanofiltration, desalination, and pervaporation

ABSTRACT

Membranes of the present disclosure possess very thin barrier layers, with high selectivity, high throughput, low fouling, and are long lasting. The membranes include graphene and/or graphene oxide barrier layers on a nanofibrous supporting scaffold. Methods for forming these membranes, as well as uses thereof, are also provided. In embodiments, an article of the present disclosure includes a nanofibrous scaffold; at least a first layer of nanoporous graphene, nanoporous graphene oxide, or combinations thereof on at least a portion of a surface of the nanofibrous scaffold; an additive such as crosslinking agents and/or particles on an outer surface of the at least first layer of nanoporous graphene, nanoporous graphene oxide, or combinations thereof.

BACKGROUND

Separation processes are important in many industries. Membraneseparation technology has been widely adapted in many separationprocesses due to its broad range of advantages, including lower energyand chemical consumption, and easier operation and maintenance. Theseprocesses include water purification, desalination, airfiltration/separation, etc.

Membranes utilized in separation processes may have varyingconfigurations, including support layers and barrier layers. Supportlayers provide the mechanical strength desirable for separationapplications. The barrier layer contributes to the primary separationprocess and determines the separation efficiency of the membrane.

Membranes used for filtration should have certain desirable properties.For example, a greater volume of material to be filtered, sometimesreferred to as throughput, is one desirable property. The ability toremove contaminants, sometimes referred to as selectivity, is anotherdesirable property. Reducing the clogging of the membranes, so that thefouling of the membranes is low and requires less frequent cleaningand/or the use of harsh chemicals or processes for cleaning, is yetanother desirable property.

Applications of electrospun nanofibrous membranes for liquid filtrationhave recently been demonstrated for microfiltration, ultrafiltration andnanofiltration. These membranes can effectively increase thepermeability of a membrane formed therefrom, while maintaining highrejection ratios.

Improved membranes and methods for forming membranes, as well as usesthereof, remain desirable.

SUMMARY

The present disclosure provides membranes suitable for filtration andsimilar separation applications. Membranes of the present disclosurepossess very thin barrier layers, with high selectivity, highthroughput, low fouling, and they are longer lasting. The membranesinclude (nanoporous) graphene and/or graphene oxide barrier layers on ananofibrous supporting scaffold. Methods for forming these membranes, aswell as uses thereof, are also provided.

In embodiments, an article of the present disclosure includes ananofibrous scaffold; at least a first layer of nanoporous graphene,nanoporous graphene oxide, or combinations thereof on at least a portionof a surface of the nanofibrous scaffold; an additive such ascross-linking agents and/or particles on an outer surface of the atleast first layer of nanoporous graphene, nanoporous graphene oxide, orcombinations thereof; and at least a second layer of nanoporousgraphene, nanoporous graphene oxide, or combinations thereof on at leasta portion of the surface of the first layer of nanoporous graphene,nanoporous graphene oxide, or combinations thereof, such that theadditive is between the first and second layers of nanoporous graphene,nanoporous graphene oxide, or combinations thereof.

In other embodiments, an article of the present disclosure includes ananofibrous scaffold; at least a first layer of nanoporous graphene,nanoporous graphene oxide, or combinations thereof on at least a portionof a surface of the nanofibrous scaffold, the first layer possessingpores having an average diameter from about 1 nm to about 1000 nm; anadditive such as cross-linking agents and/or particles on an outersurface of the at least first layer of nanoporous graphene, nanoporousgraphene oxide, or combinations thereof; and at least a second layer ofnanoporous graphene, nanoporous graphene oxide, or combinations thereofon at least a portion of the surface of the first layer of nanoporousgraphene, nanoporous graphene oxide, or combinations thereof, the secondlayer possessing pores having an average diameter from about 1 nm toabout 1000 nm, wherein the additive is between the first and secondlayers of nanoporous graphene, nanoporous graphene oxide, orcombinations thereof.

In embodiments, the nanofibrous scaffold may have a thickness of fromabout 1 μm to about 500 μm and may have voids possessing a diameter fromabout 10 nm to about 200 μm.

The first layer, the second layer, or both, may have a thickness fromabout 10 nm to about 1000 nm.

The first layer, the second layer, or both, may possess pores having anaverage diameter from about 1 nm to about 1000 nm.

The first layer, the second layer, or both, have a mean surface porosityfrom about 10% to about 70%.

The spacing between the first layer and the second layer may be fromabout 0.3 nm to about 100 nm.

Methods for forming articles having these membranes are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure will be described hereinwith reference to the following figures, wherein:

FIG. 1 includes infrared spectra of samples of graphite and grapheneoxide produced in accordance with the present disclosure; and

FIG. 2 is a graph of scans obtained by thermal gravimetric analysis onsamples of graphite and graphene oxide produced in accordance with thepresent disclosure.

DETAILED DESCRIPTION

For membrane separation technology, preparing very thin barrier layerswith high selectivity, high throughput, low fouling, and long lastingproperties are important goals. Polymeric materials widely used inmembrane separations have not yet been able to achieve such a demandingset of goals.

In accordance with the present disclosure, two-dimensional single atomiclayer materials, such as graphene and graphene oxide, especiallynanoporous graphene and nanoporous graphene oxide, are utilized toprepare separation membranes that can achieve these goals. Inembodiments, chemically modified graphene-based materials may be used asa barrier layer of a membrane possessing a nanofibrous supportingscaffold. These barrier layers can be made: very thin and strong (basedon graphene); hydrophilic, by surface functionalization includinggraphene oxide and its surface modified entities, to obtain low foulingproperties; to create two-dimensional (2D) water channels between thegraphene sheets, in order to increase the throughput of fluid(s) beingfiltered, in embodiments water; to shorten the water pathways byincreasing the surface porosity, which can be accomplished by usingnanoporous graphenes with a high surface porosity; and to seal the edgesof graphene sheets with chemical bonding, which also provides the properspacing between multiple graphene sheets.

As noted above, nanocomposite membranes of the present disclosureinclude a nanofibrous supporting scaffold. Suitable nanofibroussupporting scaffolds include, for example, structures formed of fibershaving a high surface to volume ratio, which may be from about 4 m²/m³to about 40 m²/m³, in embodiments from about 8 m²/m³ to about 20 m²/m³.In embodiments, the supporting scaffold may possess a very high specificsurface area due to fine nanofibers (having diameters from about 0.1 μmto about 1 μm, in embodiments from about 0.2 μm to about 0.8 μm), andinterconnected-pore structures.

The substrates/scaffolds can be formed of any commercially availablemicro-, ultra-, and/or nano-filtration membranes, and any of the fibrousmembranes based on electrospun nanofibers.

In embodiments, scaffolds may be made of natural and synthetic polymerswithin the purview of one skilled in the art, including, but not limitedto, polyolefins including polyethylene and polypropylene, polysulfonessuch as polyethersulfone, fluoropolymers such as polyvinylidenefluoride, polyesters including polyethylene terephthalate,polytrimethylene terephthalate, and polybutylene terephthalate,polyamides including nylon 6, nylon 66, and nylon 12, polycarbonates,polystyrenes, polynitriles such as polyacrylonitrile, polyacrylates suchas polymethyl methacrylate, polyacetates such as polyvinyl acetate,polyalcohols such as polyvinyl alcohol, polysaccharides (such aschitosan, cellulose, collagen, or gelatin), proteins such as chitin,hyaluronic acid, polyalkylene oxides such as polyethylene oxide andpolyethylene glycol, polyurethanes, polyureas, polyvinyl chloride,polyimines such as polyethylene imine, polyvinylpyrrolidone, polyacrylicacids, polymethacrylic acids, polysiloxanes such aspolydimethylsiloxane, poly(ester-co-glycol) copolymers,poly(ether-co-amide) copolymers, crosslinked forms thereof, derivativesthereof and copolymers thereof. In some embodiments, poly(acrylonitrile)(PAN), polyethersulfone (PES), polyvinylidenefluoride (PVDF),crosslinked water soluble polymers, e.g., polyvinylalcohol (PVA),modified cellulose and modified chitosan, their chemical derivativesand/or copolymers, may be utilized. Combinations of the foregoing mayalso be used to form suitable scaffolds.

In some embodiments, it may be desirable to crosslink fluid-solublepolymers to form a nanofibrous scaffold. For example, water-solublepolymers, such as polyvinyl alcohol, polysaccharides (including chitosanand hyaluronan), polyalkylene oxides (including polyethylene oxide),gelatin, and their derivatives may be crosslinked to render thesepolymers suitable for use as a nanofibrous scaffold. Crosslinking may beconducted using methods within the purview of those skilled in the art,including the use of crosslinking agents. Suitable crosslinking agentsinclude, but are not limited to, C₂-C₈ dialdehyde, C₂-C₈ diepoxy, C₂-C₈monoaldehydes having an acid functionality, C₂-C₉ polycarboxylic acids,combinations thereof, and the like. These compounds are capable ofreacting with at least two hydroxyl groups of a water-soluble polymer.

Other suitable crosslinking methods include conventional thermal-,radiation- and photo-crosslinking reactions within the purview of thoseskilled in the art. Two important criteria for the selection of acrosslinking agent or method are as follows: (1) the crosslinking agentor method should not dissolve the nanofibrous scaffold layer; and (2)the crosslinking agent or method should not induce large dimensionalchange, e.g., hydrophilic electrospun nanofibrous scaffold layers maydisplay very large shrinkage in hydrophobic solvents such ashydrocarbons because of their hydrophilic nature.

Other examples of suitable crosslinking agents which may be utilized informing a nanofibrous scaffold include, but are not limited to,glutaraldehyde, 1,4-butanediol diglycidyl ether, glyoxal, formaldehyde,glyoxylic acid, oxydisuccinic acid, citric acid, fumaric acid,combinations thereof, and the like. In some embodiments, it may beuseful to treat polyvinyl alcohol with a crosslinking agent such asglutaraldehyde.

The amount of crosslinking agent added to the polymer, such as polyvinylalcohol, may vary from about 0.1 to about 10 percent by weight of thecombined weight of the crosslinking agent and polymer, in embodimentsfrom about 0.5 to about 5 percent by weight of the combined weight ofthe crosslinking agent and polymer.

In forming the nanofibrous scaffold of the present disclosure, thepolymer is often first placed in a solvent, such as N,N-dimethylformamide (DMF), tetrahydrofuran (THF), methylene chloride, dioxane,ethanol, propanol, butanol, chloroform, water, or combinations of thesesolvents, so that the polymer is present at an amount from about 1 toabout 40 percent by weight of the polymer solution, in embodiments fromabout 3 to about 25 percent by weight of the polymer solution, inembodiments from about 5 to about 15 percent by weight of the polymersolution.

In some embodiments, it may be desirable to add a surfactant or anothersolvent-miscible liquid to the polymer solution utilized to form thenanofibrous scaffold. The surfactant or other solvent-miscible liquidmay lower the surface tension of the solution, which may help stabilizethe polymer solution during electro-spinning, electro-blowing, and thelike. Suitable surfactants include, for example, octylphenoxypolyethoxyethanol (commercially available as TRITON X-100), sorbitan monolaurate,sorbitan sesquioleate, glycerol monostearate, polyoxyethylene,polyoxyethylene cetyl ether, dimethyl alkyl amines, methyl dialkylamines, combinations thereof, and the like. Where utilized, thesurfactant may be present in an amount from about 0.001 to about 10percent by weight of the polymer solution, in embodiments from about0.05 to about 5 percent by weight of the polymer solution, inembodiments from about 0.1 to about 2 percent by weight of the polymersolution. The solvent miscible fluid with the solvent forms a solventmixture that can dissolve the polymer but changes the surface tension ofthe polymer solution and the evaporation rate of the solvent mixture.

In embodiments, the nanofibrous scaffold may be fabricated usingelectro-spinning, electro-blowing, blowing-assisted electro-spinning,and/or solution blowing technologies. Electro-spinning processes usemainly electric force, but often without the assistance of gas flow. Tothe contrary, solution blowing processes use only gas flow, without theuse of electric force. Blowing-assisted electro-spinning andelectro-blowing both use electric force and gas-blowing shear forces. Inblowing-assisted electro-spinning processes, the electric force is thedominating factor, while the gas-blowing feature can assist in shearingthe fluid jet stream and in controlling the evaporation of the solvent(lower throughput, smaller diameter). In contrast, in electro-blowingprocesses the gas-blowing force is the dominating factor to achieve thedesired spin-draw ratio, while the electric force may enable furtherelongation of fibers (higher throughput, larger diameter).

The thickness of the nanofibrous scaffold may vary from about 1 μm toabout 500 μm, in embodiments from about 10 μm to about 300 μm, inembodiments from about 30 μm to about 150 μm in thickness. In someembodiments, the thickness of the scaffold is from about 40 μm to about50 μm.

The nanofibrous scaffold possesses pores or voids which assist in thefunctioning of the membranes of the present disclosure. The diameter ofthese voids may be from about 10 nm to about 200 μm, in embodiments fromabout 50 nm to about 30 μm, in embodiments from about 100 nm to about 10μm. In some embodiments, the pore size may be from about 0.2 μm to about0.6 μm.

Aside from nanofibers which can be fabricated by electrospinning,nanofibers can also be extracted from natural polysaccharides (e.g.,cellulose and chitin). Such polysaccharide nanofibers can have highdegrees of crystallinity and are therefore inert over large ranges of pHvalues. These nanofibers have nominal diameters ranging from a fewnanometers to tens of nanometers, essentially overlapping withnanofibers normally fabricated by electro-spinning These (nano-)fibersuspensions can also be fabricated to form a support layer for the(nanoporous) graphene or graphene oxide barrier layer.

As noted above, in embodiments a membrane of the present disclosureincludes a barrier layer of graphene applied to the nanofibrousscaffold. As used herein, graphene includes both graphene andcompounds/compositions based upon graphene, including graphene oxide.Graphene is a single layer of carbon atoms with many useful properties.Graphene may be obtained from graphite by mechanical exfoliation, byepitaxial growth, and reduction of either silicon carbide or grapheneoxide.

Graphene oxide (hereinafter referred to, in embodiments, as “GO”) is anelectrically insulating material composed of a single layer of graphiteoxide, the reduction precursor of graphene, having oxygen functionalgroups bonded to the graphene basal-plane. Graphene oxide can beobtained from the exfoliation of graphite oxide. Similar to graphene, GOalso possesses one-atom thick sheets with high surface-to-volume ratioand extraordinary physical properties. Compared with graphene, GOpossesses functional groups, including hydroxyl, epoxy, and/orcarboxylic groups, on both the edge and surface of the two-dimensionalplane, which are the result of oxidation reactions. These functionalgroups may provide more opportunities for further chemicalmodifications.

For example, poly (vinyl alcohol) (PVA) could be linked to grapheneoxide sheets via the reaction between hydroxyl groups of PVA andcarboxylic acid groups on GO. The resulting material can becharacterized by using solution-phase techniques. The carboxylic acidgroups on GO could also react with polyethylene glycol (PEG) to formcomposites of PEG and GO, which could be employed as drug carriers forcancer treatment and as intracellular imaging probes. The epoxy groupson GO can undergo ring-opening reactions with NH₂-terminated ionicliquids (1-(3-aminopropyl)-3-methylimidazolium bromide), with theproduct being well dispersed in DMF, DMSO, and water. The hydroxylgroups on GO can also be used for chemical modifications. For example,diboronic acid could crosslink the GO sheets by forming bonds withhydroxyl groups on the GO surface to produce a three-dimensional GOframework for hydrogen storage.

Due to oxygen functional groups such as carboxyls, epoxides, andalcohols, GO is hydrophilic and can readily exfoliate as single sheetswhen ultrasonicated in water. The average size of an individual GOsheet, dispersed in water, may be about 1 μm², with a thickness of fromabout 1 nm to about 1.5 nm.

GO membranes, having a layer thickness in the submicron range, allowunimpeded permeation of water vapor, but are totally impermeable toother liquid and gas molecules, including helium. This unique behaviormay be attributed to the phenomenon of “nanocapillaries” formed betweenthe flakes of GO that make up the GO sheets. In other words, thehydrophilic nature of carboxylic acid groups and hydroxyl groups (whichattract water molecules) may create a tortuous network of confined waterchannels, allowing the transport of water molecules, but preventing thediffusion of larger size and hydrophobic molecules.

Methods for producing graphene oxide, as noted above, are within thepurview of those skilled in the art. Graphene oxide is also commerciallyavailable, for example, from Graphene Supermarket, Inc. (Calverton,N.Y.).

Graphene-based materials may be desirable for barrier layers because oftheir single atomic layer structure, which can provide the possibilityof making ultra-thin barrier layers from multiple layers of graphene.These thin barrier layers exhibit high flux. It is believed onemechanism by which these graphene-based materials are able to separatecontaminants from a fluid stream is the diffusion of the permeate beingcarried out via the channels between the graphene (or graphene oxide)nanosheets. In other words, the permeate travels through the interlayerspaces built by graphene or graphene oxide layers. Although multiplegraphene and graphene oxide nanosheets could build up very thin barrierlayers, the permeate still needs to travel a tortuous path between thenano-sheets to arrive at the other end of the membrane, thereby limitingfurther improvements of membrane efficiency. Thus, the use of nanoporousgraphene/graphene oxide can shorten the 2D pathways between the graphenesheets and improve the fluid transport efficiency of a membraneincluding these materials.

In accordance with the present disclosure, an approach to counter thisdilemma has been developed, using graphene or graphene oxide nanosheetswith nano-pores on the surface; the assembled barrier layer has morechannels (or shorter pathways) than ones built by non-porous layers. Thesingle atomic layer structure of graphene-based materials provides thepossibility of fabricating an ultra-thin barrier layer from multiplelayers of graphene and/or GO. nanoporous surfaces allow higherpermeability. By precisely controlling the parameters duringfabrication, the barrier layer thickness and the assembly of thegraphene nano-sheets can be well tuned. The resulting separationmembranes have higher flux and higher rejection ratios than existingcommercial products and are useful in many applications.

Different treatments may be employed to produce and adjust the pore sizeof nanoporous graphene nanosheets. Small pore sizes can be created byoxidatively etching graphene nanosheets, while graphene dots can beremoved by H₂O₂ treatment. As a result, nanoscale pores can begenerated.

Porous graphenes can also be fabricated by other approaches. One is touse thermal treatment to generate pores on the graphene oxide plane.Heating at 140° C. could facilitate decomposition of graphene oxide,during which carbon dioxide and carbon monoxide would be released fromthe oxygen-containing sites on graphene oxide. The thermal treatmentcould be carried out either before or after the assembly of grapheneoxide nanosheets. The graphene oxide layers may be heated to from about130° C. to about 140° C. for a short time, e.g., less than 5 minutes.

Additional approaches to form nanoporous graphenes include the use ofion bombardment together with chemical etching to produce nanopores on agraphene sheet. Single layers of graphene may be irradiated by highdensity ion beams to generate very small defects. The spots with defectsare then oxidized by a chemical agent such as hydrogen peroxide and growto nanopores. The pore density is dependent on the density of the ionbeam, and it could be as high as 10¹² pores per cm². The pores producedby this method could have sizes less than 1 nm and the pore sizedistribution is relatively narrow, in embodiments from about 0.5 nm toabout 5 nm.

In embodiments, single-layer graphene may be deposited on a grid andirradiated by gallium ions at a density of about 6×10¹² ions/cm² with 8kV acceleration voltage and 52° incident angle. Then the graphene can betreated in potassium permanganate (1.875 mM KMnO₄ in 6.25% H₂SO₄) for120 minutes. This method can produce nanoporous graphene with relativelylow surface porosity.

In embodiments of the present disclosure, electrophoretic deposition canalso be used to create nanoporous graphene oxide nanosheets. Theelectric field could be generated by an applied voltage and the coatingmaterials can be deposited onto a substrate under the influence of anapplied electric field.

Electrophoretic deposition is a process where colloidal particles, suchas polymers, pigments, dyes, ceramics and metals, suspended in a liquidphase, can be deposited onto an electrode in an electric field.Electrophoretic deposition includes electrophoretic coating,electrophoretic painting, electrocoating, cathodic electrodeposition,and anodic electrodeposition.

Methods and apparatus for electrophoretic deposition are within thepurview of one skilled in the art. Electrophoretic deposition means thatcharged particles (such as for a suspension of colloidal particleshaving a net surface charge) are moving in response to an appliedexternal electric field (electrophoresis) resulting in the deposition ofmaterial onto an electrode. The suspension or colloid is produced byplacing the particles of interest, in this case graphene oxide, into asuitable solvent for formation of the suspension or colloid.

Suitable solvents for forming the suspension/colloid include, but arenot limited to, water, ethylene glycol, dimethylformamide (DMF),n-methylpyrrolidone (NMP), tetrahydrofuran (THF), combinations thereof,and the like. The graphene oxide may be present in the solution inamounts from about 1×10⁻¹° % by weight of the solution to about 10% byweight of the solution, depending on the mean size of exfoliatedgraphene oxide sheets, in embodiments from about 5×10⁻⁸% by weight ofthe solution to about 0.5% by weight of the solution.

The concentration of graphene oxide in the resulting suspension orcolloid may be, in embodiments, from about 5×10⁻⁶% to about 5×10⁻²%, inembodiments from about 5×10⁻⁴% to about 5×10⁻³%. The resulting colloidor suspension is placed in a deposition tank having a working electrode,and counter electrode. Voltage is applied between the working electrodeand counter electrode.

Suitable voltages applied to the suspension/colloid in the depositiontank may be from about 1 V to about 300 V, in embodiments from about 10V to about 100 V, for a period of time from about 10 seconds to about 30minutes, in embodiments from about 60 seconds to about 10 minutes.

In embodiments, electrophoretic deposition may be used to produceuniform structures having a coating thickness from about 10 nm to about100 nm, which can be easily controlled by adjusting the coating time andthe applied voltage. For example, GO shows negative charge with a zetapotential of −64.7 mV and it could be deposited on indium tin oxide(ITO) electrodes at a voltage of 150 V for a period of time of about 45seconds.

The resulting layer of nanoporous graphene oxide may have a thicknessfrom about 10 nm to about 1000 nm, in embodiments from about 20 nm toabout 500 nm. In some embodiments, the thickness may be from about 10 nmto about 20 nm.

The layer of nanoporous graphene oxide may possess pores having anaverage diameter from about 1 nm to about 1000 nm, in embodiments fromabout 5 nm to about 300 nm. In accordance with the present disclosure,the layer of nanoporous graphene oxide may have a mean surface porosityfrom about 10% to about 70%, in embodiments from about 40% to about 60%.

Deposition of nanoporous graphene/graphene oxide layers on top ofpolymeric substrates could occur, in embodiments, by vacuum filtration.The nanoporous graphene/graphene oxide sheets may be dispersed intowater, with sonication used to facilitate this dispersion. Then, thegraphene/graphene oxide suspension (in embodiments, about 5 ppm) may betransferred into a vacuum filtration funnel, below which a polymericsubstrate (such as GSWP0.22, commercially available from Millipore) isinstalled. A vacuum is applied underneath the substrate in order todrive the suspension in the funnel to slowly and evenly flow through thesubstrate. Since the pore size of the substrate may be smaller than thesize of nanoporous graphene/graphene oxide sheets, the graphene/grapheneoxide sheets remain on the surface of the substrate. By controlling theconcentration of the suspension (from 0.5 ppm to 5 ppm) and thefiltration flow rate, the deposition thickness of nanosheets may becontrolled.

Spin-coating is another approach for deposition of nanoporousgraphene/graphene oxide nano-sheets. Here, a suspension of nanoporousgraphene/graphene oxide, in embodiments about 500 ppm, may be preparedbefore spin-coating. The polymeric substrate may be taped onto therotating plate of a spin-coating machine. The spinning speed may be setat about 3000 rpm. The suspension may then be dropped onto the center ofthe spinning substrate. After the droplets spread on the substrate, theabove steps may be repeated, for example, about 3 times. The centrifugalforce produced by spin-coating and the quick evaporation of waterfacilitates an ordered assembly of nanoporous graphene/graphene oxidenanosheets.

As noted above, in embodiments, barrier layers may be formed frommultiple layers of nanoporous graphene/graphene oxide, with someadditional treatment or manipulation to enforce or enhance themechanical stability of the resulting nanoporous graphene sheets. Forexample, the nanoporous graphene may be subjected to chemicalmodifications by the inclusion of additives to adjust (i) the spacingsbetween graphene sheets, and (ii) the seal on the edges of graphenesheets used to stabilize the barrier layer.

In embodiments, enforcing the mechanical stability of assemblednanoporous graphene (or graphene oxide) sheets may occur as follows. Thenanoporous graphene (or graphene oxide) sheets may be held togethermainly by van der Waals interactions, with the Young's modulus of thefilm being around 50 GPa. In order to increase the stability andstiffness of the nanoporous graphene (or graphene oxide) layer, but notto substantially expand the inter-sheet spacing, small molecules couldbe employed as additives to cross-link the nanoporous graphene oxidesheets used to form a barrier layer.

For example, in embodiments, boric acid may be used to form covalentbonds with the oxygen-containing functional groups on graphene oxide andfurther cross-linked to the nanoporous graphene oxide sheets forming thebarrier layer, to improve their mechanical properties. The Young'smodulus for graphene oxide film after boric acid cross-linking could beas high as 127 GPa, and the ultimate tensile strength may be increasedto 185 GPa. Other functional molecules, such as ethylenediamine,butylenediamine, p-phenylenediamine, glutaraldehyde, epichlorohydrin,phosphorus oxychloride (phosphoryl chloride), trimetaphosphate, linearmixed anhydrides of acetic and di- or tribasic carboxlic acids, vinylsulfone, diepoxides, cyanuric chloride, aldehydes (for example,formaldehyde, acetaldehyde, acrolein), 1,4-butanediol diglycidyl ether,glyoxal, glyoxylic acid, oxydisuccinic acid, citric acid, fumaric acid,combinations thereof, and the like, could also serve as cross-linkers,depending on the spacing needs. Use of these cross-linking agentsresults, in embodiments, in the formation of channels between thegraphene/graphene oxide sheets that are just wide enough to permit watermolecules to pass through.

Where used, cross-linking agents may be in solution including suitablesolvents such as water, ethylene glycol, dimethylformamide (DMF),n-methylpyrrolidone (NMP), tetrahydrofuran (THF), combinations thereofand the like, at a concentration from about 0.001% to about 50%, inembodiments from about 0.01% to about 10% .

In embodiments, a nanoporous graphene/graphene oxide nanosheet isapplied to a substrate, followed by application of a cross-linkingagent, followed by application of at least one additional nanoporousgraphene/graphene oxide nanosheet, so that the additive (herecross-linking agent) is between the nanosheets.

In accordance with the present disclosure, the interlayer spacing ofassembled nanoporous graphene/graphene oxide nanosheets may also beadjusted. The spacing between nanoporous graphene (or graphene oxide)layers plays an important role in the separation process, since the flowof the permeate mainly takes place in the 2D channels between thenanoporous graphene (or graphene oxide) nanosheets. Therefore, inembodiments, the spacing between nanoporous graphene (or graphene oxide)sheets may be adjusted depending on different membrane applications, forexample, whether the membrane is to be used for RO or nanofiltration.

Increasing the spacing between nanoporous graphene (or graphene oxide)sheets may be accomplished as follows. The enlargement of spacingbetween nanoporous graphene (or graphene oxide) sheets can be achievedwith particles as additives. For example, the enlargement of spacingbetween nanoporous graphene (or graphene oxide) sheets can be achievedby sandwiching appropriate particles including molecules, nanoparticles,or polymers between nanoporous graphene (or graphene oxide) sheets, inorder to increase or decrease the spacing as desired. Small moleculeswith functional groups, such as borate, ethylenediamine,epichlorohydrin, phosphorus oxychloride (phosphoryl chloride),trimetaphosphate, linear mixed anhydrides of acetic and di- or tribasiccarboxlic acids, vinyl sulfone, diepoxides, cyanuric chloride, aldehydes(for example, formaldehyde, glutaraldehyde (GA), acetaldehyde,acrolein), 1,4-butanediol diglycidyl ether, glyoxal, glyoxylic acid,oxydisuccinic acid, citric acid, fumaric acid, combinations thereof, andthe like, can be placed between layers of graphene (or graphene oxide)nanosheets and can form covalent bonds with the graphene (or grapheneoxide) nanosheets, thereby adjusting the spacing between the nanosheets.

Other molecules, nanoparticles and/or polymers that may be placedbetween the nanoporous graphene (or graphene oxide) nanosheets include,for example, polyolefins including polyethylene and polypropylene,polysulfones such as polyethersulfone, fluoropolymers such aspolyvinylidene fluoride, polyesters including polyethyleneterephthalate, polytrimethylene terephthalate, and polybutyleneterephthalate, polyamides including nylon 6, nylon 66, and nylon 12,polycarbonates, polystyrenes, polynitriles such as polyacrylonitrile,polyacrylates such as polymethyl methacrylate, polyacetates such aspolyvinyl acetate, polyalcohols such as polyvinyl alcohol,polysaccharides (such as chitosan, cellulose, collagen, or gelatin),proteins such as chitin, hyaluronic acid, polyalkylene oxides such aspolyethylene oxide and polyethylene glycol, polyurethanes, polyureas,polyvinyl chloride, polyimines such as polyethylene imine,polyvinylpyrrolidone, polyacrylic acids, polymethacrylic acids,polysiloxanes such as polydimethylsiloxane and polyhedral oligomericsilsesquioxane (POSS), poly(ester-co-glycol) copolymers, silicananoparticles, carbon nanotubes, carbon nanofibers, poly(ether-co-amide)copolymers, crosslinked forms thereof, derivatives thereof andcopolymers thereof. In some embodiments, poly(acrylonitrile) (PAN),polyethersulfone (PES), polyvinylidenefluoride (PVDF), crosslinked watersoluble polymers, e.g., polyvinylalcohol (PVA), modified cellulose andmodified chitosan, their chemical derivatives and/or copolymers,combinations thereof, and the like, may be used as additives. Theamounts of these additives applied to the nanosheets will vary dependingupon the intended use of the resulting membrane.

Similar to the application of cross-linking agents to the of nanoporousgraphene/graphene oxide nanosheets, a nanoporous graphene/graphene oxidenanosheet is applied to a substrate, followed by application of anadditive particle (and optional cross-linking agent), followed byapplication of at least one additional nanoporous graphene/grapheneoxide nanosheet.

In embodiments, the spacing between nanoporous graphene (or grapheneoxide) layers could be increased from about 0.3 nm to about 100 nm, inembodiments from about 0.5 nm to about 10 nm, in embodiments about 0.7nm. As some spacings may only be large enough for the passage of watervapor, the spacing should be at least large enough for water passage inapplications for desalination, for example. Polyelectrolytes andnanoparticles such as polyvinylamine and cellulose nanofibers can beused to obtain larger distances between nanoporous graphene (or grapheneoxide) sheets. However, the sandwiched structure, built by alayer-by-layer assembly method, may have difficulties for scale upoperation.

Decreasing the spacing between nanoporous graphene oxide layers can becarried out by partially reducing the nanoporous graphene oxide tonanoporous graphene. It may be difficult to decrease spacing betweentightly packed pristine graphene layers, so only narrowing the spacingbetween graphene oxide layers may be an alternative. Tightly packedgraphene oxide layers have the interspacing larger than tightly packedpure graphene layers, because of the functional groups on the grapheneoxide surface, which can create more space between graphene oxidesheets. Therefore, the effective way to decrease the interspacingbetween nanoporous graphene oxide sheets could be to partially reducethe nanoporous graphene oxide.

In embodiments, different physical and chemical reducing approaches,such as high temperature treatment (>140° C.) under nitrogen atmosphere,and conducting the reduction reaction with hydrazine, could be used toreduce graphene oxide.

Membranes including the nanofibrous scaffold and barrier layer havingmore than one sheet of nanoporous graphene/graphene oxide thereon mayhave a permeate flux of from about 10 kg/m²hr to about 500 kg/m²hr, inembodiments from about 50 kg/m²hr to about 100 kg/m²hr. The membraneincluding the nanofibrous scaffold and nanoporous graphene oxide barrierlayer thereon may have a separation factor from about 95% to about99.9%, in embodiments from about 97.5% to about 99%.

The new class of nanocomposite membranes in accordance with the presentdisclosure, including cross-linked nanoporous graphene or graphene oxidebarrier layers on a nanofibrous supporting scaffold, have highefficiency for nanofiltration, reverse osmosis (RO), forward osmosis(FO) and pervaporation. Two key factors for these uses include: (1) thepore sizes and porosity of nanoporous graphene or graphene oxide; and(2) the spacings between the graphene nanosheets as adjusted bycross-linking agent(s) or inclusion of particulate additives, which canalso stabilize the nanoporous graphene-based sheets.

The advantages of the new class of graphene-based membranes, based on ananoporous graphene (oxide) barrier layer and a supporting scaffold,include the following.

-   -   (1) Nanoporous graphene and graphene oxide nanosheets with        adjustable pore sizes and porosity can be fabricated by        oxidatively treating graphene or graphene oxide. The pore sizes        can go down to 0.5 nm or lower to fit nanofiltration,        desalination, and/or pervaporation applications. Nanoporous        graphenes with relatively high surface porosity and 2D channels        between the graphene sheets are used to permit water passage and        for excluding other unwanted contaminants.    -   (2) The spacings between nanosheets can also be tuned by        selecting different cross-linking reagents, such as boric acid,        glutaraldehyde, and epichlorohydrin, to create channels,        essentially just wide enough for water molecules to pass        through.    -   (3) By the combination of nanoporous graphenes to short cut the        water pathways, and chemical cross-linking to stabilize the        nanoporous graphene sheets and to provide the proper spacing        between the nanoporous graphene sheets, the graphene-based        barrier layer, together with the nanofibrous composite        substrate, can form a new separation membrane for        nanofiltration, reverse osmosis (RO) (desalination) forward        osmosis (FO) (energy generation) and pervaporation.

In accordance with the present disclosure, both nanopores on graphenesheets and nano-spacings between nanoporous graphene nanosheets can beused, respectively, as ‘1D’ pores and 2D ‘channels’ for liquid water.Therefore, a new form of membrane with a very thin barrier layerthickness can be fabricated for further improvement of permeability andretention.

The following Examples are being submitted to illustrate embodiments ofthe present disclosure. These Examples are intended to be illustrativeonly and are not intended to limit the scope of the present disclosure.Also, parts and percentages are by weight unless otherwise indicated. Asused herein, “room temperature” refers to a temperature of from about20° C. to about 30° C.

EXAMPLE 1

In accordance with the present disclosure, the fabrication of membranesusing nanoporous graphene layers was carried out and the application ofsuch membranes for water purification and pervaporation weredemonstrated.

Graphene oxide nanosheets were prepared from graphite powder. About 2grams of graphite powder and about 12 grams of KMnO₄ were added into 300mL of H₂SO₄ and H₃PO₄ at a ratio of 9/1 (v/v). The reaction mixture wasstirred at 50° C. for 12 hours. The resulting mixture was cooled down toroom temperature and was mixed with 400 grams of ice and 2 mL of 30%H₂O₂. The graphite oxide was washed with water until neutral (pH 7). Thesuspension of graphene oxide (500 ppm) was obtained from graphite oxidewith a sonication (homogenizer) treatment.

EXAMPLE 2

Preparation of nanoporous graphene/graphene oxide nanosheets.Graphene/graphene oxide (commercially available materials) (100 mg) weredispersed into 100 mL H₂O₂ (30%) and the mixture was sonicated for 10minutes at room temperature to achieve a homogenous dispersion. Then,the mixture was heated to 70° C. and kept at that temperature for 72hours. (The time period can vary depending on the pore size and porositydesired.) After 72 hours, the mixture was dialyzed in water with adialysis tube (MWCO: 15 kDa) for 3 days. The produced nanoporousgraphene oxide had a pore size of about 3 nm to about 5 nm.

EXAMPLE 3

Infrared spectrometer (Thermo Scientific, Nicolet iS10) was used todetermine the IR spectra of the samples, as shown in FIG. 1. As seen inFIG. 1, the two peaks in the 1200 cm⁻¹ region were not expected to bepresent in the spectra since they did not correspond to any carbon tocarbon bonds. They are most likely due to some organic impuritiespresent in the original material. Many peaks in the spectrum of grapheneoxide appeared, including the corresponding peaks of CO (1644 cm⁻¹),COOH (1746 cm⁻¹) and OH (3487 cm⁻¹), showing that the graphene sheetshad been successfully oxidized to graphene oxide sheets.

EXAMPLE 4

Thermal gravimetric analysis on the samples was carried out using a TGAsystem (Perkin Elmer, TGA 7) under the following conditions. The gastype was set to be air, with a flow rate of 20 ml/min. The initialtemperature was set at 50° C. An isothermal scan was carried out byholding the temperature at 50° C. for 10 minutes and then thetemperature scan was followed by raising the temperature to 800° C. witha raising rate of 10° C./min. The results are shown in FIG. 2.

As can be seen in FIG. 2, compared with graphite, the thermal stabilityof graphene oxide (GO) and nanoporous GO decreased, due to the existenceof hydroxyl, carboxyl, epoxy, and carbonyl groups generated from theoxidation of graphite with the oxidized system.

EXAMPLE 5

Deposition of nanoporous graphene/graphene oxide layers on top ofpolymeric substrates. The vacuum filtration process was as follows. Thenanoporous graphene/graphene oxide sheets were dispersed into water,with sonication used to facilitate this dispersion. Then, thegraphene/graphene oxide suspension (typically, 5.0 ppm) was transferredinto a vacuum filtration funnel, below which the polymeric substrate(GSWP0.22, Millipore) had been installed. A vacuum was appliedunderneath the substrate in order to drive the suspension in the funnelto flow, slowly and evenly, through the substrate. Since the pore sizeof the substrate was 0.22 microns, which was smaller than the size ofnanoporous graphene/graphene oxide sheets, the graphene/graphene oxidesheets remained on the surface of the substrate. By controlling theconcentration of the suspension (from 0.5 ppm to 5.0 ppm) and thefiltration flow rate, the deposition thickness of nanosheets wascontrolled. In this example, it was of the order of 100 nm.

EXAMPLE 6

Water purification is one of the potential applications of the membranesof the present disclosure. In this Example, a Millipore stainless steel(dead-end) cell (25 mm diameter) with an effective filtration area of4.9 cm² was used for the filtration test for membranes includinggraphene oxide and nanoporous graphene oxide, as described in Example 2.The feeding solution had 0.02 μm particles (100 ppm) in water. Duringthe filtration process, the feeding solution was pumped to circulateinside the system. The permeability and rejection were calculated byusing the following equations:

$\begin{matrix}{P = \frac{\Delta \; V}{\Delta \; {T \cdot \Delta}\; {P \cdot S}}} & (1)\end{matrix}$

Where ΔV is the permeate volume accumulated during a time interval Δt, Sis the surface area of the membrane, and ΔP is the applied pressureacross the membrane. The rejection ratio, R, is

$\begin{matrix}{{R\mspace{14mu} \%} = {{\frac{C_{feed} - C_{permeate}}{C_{feed}}100}\%}} & (2)\end{matrix}$

Where Cfeed and Cpermeate are the concentrations of the feed solutionand the permeate solution, respectively.

The filtration results are listed in Table 1 below. The flux of thenanoporous graphene oxide-based membrane was about 2 times higher thanthat of the graphene oxide-based membrane, while the rejection ratiosremained the same.

TABLE 1 Filtration performance of membranes with a graphene oxide- and ananoporous graphene oxide-based barrier layer. Membranes Flux(L/m2h/psi) Rejection ratio (%) GO 1.4 99.9 Porous GO 3.6 99.9

EXAMPLE 7

Spin-coating is another approach for deposition of nanoporousgraphene/graphene oxide nano-sheets for demonstration purposes. Asuspension of nanoporous graphene/graphene oxide (500 ppm) was preparedbefore spin-coating. The polymeric substrate was taped onto the rotatingplate of the spin-coating machine. The spinning speed was set at about3000 rpm. About 1 mL of the suspension was dropped onto the center ofthe spinning substrate. After the droplet was spread on the substrate,the above steps were repeated 3 times. The centrifugal force produced byspin-coating and the quick evaporation of water facilitated an orderedassembly of the nanoporous graphene/graphene oxide nanosheets.

It will be understood that various modifications may be made to theembodiments disclosed herein. Therefore, the above description shouldnot be construed as limiting, but merely as an exemplification ofpreferred embodiments. Those skilled in the art will envision othermodifications within the scope and spirit of the present disclosure.Such modifications and variations are intended to come within the scopeof the following claims.

What is claimed is:
 1. An article comprising: a nanofibrous scaffold; atleast a first layer of nanoporous graphene, nanoporous graphene oxide,or combinations thereof on at least a portion of a surface of thenanofibrous scaffold; an additive selected from the group consisting ofcross-linking agents and particles on an outer surface of the at leastfirst layer of nanoporous graphene, nanoporous graphene oxide, orcombinations thereof; and at least a second layer of nanoporousgraphene, nanoporous graphene oxide, or combinations thereof on at leasta portion of the surface of the first layer of nanoporous graphene,nanoporous graphene oxide, or combinations thereof, such that theadditive is between the first and second layers of nanoporous graphene,nanoporous graphene oxide, or combinations thereof.
 2. The article ofclaim 1, wherein the nanofibrous scaffold comprises a polymer selectedfrom the group consisting of polyolefins, polysulfones, fluoropolymers,polyesters, polyamides, polycarbonates, polystyrenes, polynitriles,polyacrylates, polyacetates, polyalcohols, polysaccharides, proteins,polyalkylene oxides, polyurethanes, polyureas, polyimines, polyacrylicacids, polymethacrylic acids, polysiloxanes, poly(ester-co-glycol)copolymers, poly(ether-co-amide) copolymers, derivatives thereof andcopolymers thereof.
 3. The article of claim 1, wherein the nanofibrousscaffold comprises a polymer selected from the group consisting ofpolyethylene, polypropylene, polyethersulfone, polyvinylidene fluoride,polyethylene terephthalate, polytrimethylene terephthalate, polybutyleneterephthalate, nylon 6, nylon 66, nylon 12, polystyrene,polyacrylonitrile, polymethyl methacrylate, polyvinyl acetate, polyvinylalcohol, chitosan, cellulose, collagen, gelatin, polyethylene oxide,polyethylene glycol, polyvinyl chloride, polyethylene imine,polyvinylpyrrolidone, polydimethylsiloxane, derivatives thereof andcopolymers thereof.
 4. The article of claim 1, wherein the nanofibrousscaffold has a thickness of from about 1 μm to about 500 μm.
 5. Thearticle of claim 1, wherein the nanofibrous scaffold has voidspossessing a diameter from about 10 nm to about 200 μm.
 6. The articleof claim 1, wherein the first layer, the second layer, or both, may havea thickness from about 10 nm to about 1000 nm.
 7. The article of claim1, wherein the first layer, the second layer, or both, may possess poreshaving an average diameter from about 1 nm to about 1000 nm.
 8. Thearticle of claim 1, wherein the first layer, the second layer, or both,have a mean surface porosity from about 10% to about 70%.
 9. The articleof claim 1, wherein the cross-linking agent is selected from the groupconsisting of boric acid, ethylenediamine, butylenediamine,p-phenylenediamine, glutaraldehyde, epichlorohydrin, phosphorusoxychloride, trimetaphosphate, linear mixed anhydrides of acetic and di-or tribasic carboxlic acids, vinyl sulfone, diepoxides, cyanuricchloride, aldehydes, formaldehyde, acetaldehyde, acrolein,1,4-butanediol diglycidyl ether, glyoxal, glyoxylic acid, oxydisuccinicacid, citric acid, fumaric acid, and combinations thereof.
 10. Thearticle of claim 1, wherein the particles are selected from the groupconsisting of cellulose nanofibers, polyhedral oligomericsilsesquioxane, silica nanoparticles, carbon nanotubes, carbonnanofibers, and combinations thereof.
 11. The article of claim 1,wherein the spacing between the first layer and the second layer is fromabout 0.3 nm to about 100 nm.
 12. An article comprising: a nanofibrousscaffold; at least a first layer of nanoporous graphene, nanoporousgraphene oxide, or combinations thereof on at least a portion of asurface of the nanofibrous scaffold, the first layer possessing poreshaving an average diameter from about 1 nm to about 1000 nm; an additiveselected from the group consisting of cross-linking agents and particleson an outer surface of the at least first layer of nanoporous graphene,nanoporous graphene oxide, or combinations thereof; and at least asecond layer of nanoporous graphene, nanoporous graphene oxide, orcombinations thereof on at least a portion of the surface of the firstlayer of nanoporous graphene, nanoporous graphene oxide, or combinationsthereof, the second layer possessing pores having an average diameterfrom about 1 nm to about 1000 nm, wherein the additive is between thefirst and second layers of nanoporous graphene, nanoporous grapheneoxide, or combinations thereof.
 13. The article of claim 12, wherein thenanofibrous scaffold comprises a polymer selected from the groupconsisting of polyolefins, polysulfones, fluoropolymers, polyesters,polyamides, polycarbonates, polystyrenes, polynitriles, polyacrylates,polyacetates, polyalcohols, polysaccharides, proteins, polyalkyleneoxides, polyurethanes, polyureas, polyimines, polyacrylic acids,polymethacrylic acids, polysiloxanes, poly(ester-co-glycol) copolymers,poly(ether-co-amide) copolymers, derivatives thereof and copolymersthereof.
 14. The article of claim 12, wherein the nanofibrous scaffoldcomprises a polymer selected from the group consisting of polyethylene,polypropylene, polyethersulfone, polyvinylidene fluoride, polyethyleneterephthalate, polytrimethylene terephthalate, polybutyleneterephthalate, nylon 6, nylon 66, nylon 12, polystyrene,polyacrylonitrile, polymethyl methacrylate, polyvinyl acetate, polyvinylalcohol, chitosan, cellulose, collagen, gelatin, polyethylene oxide,polyethylene glycol, polyvinyl chloride, polyethylene imine,polyvinylpyrrolidone, polydimethylsiloxane, derivatives thereof andcopolymers thereof.
 15. The article of claim 12, wherein the nanofibrousscaffold has a thickness of from about 1 μm to about 500 μm.
 16. Thearticle of claim 12, wherein the nanofibrous scaffold has voidspossessing a diameter from about 10 nm to about 200 μm.
 17. The articleof claim 12, wherein the first layer, the second layer, or both, mayhave a thickness from about 10 nm to about 1000 nm.
 18. The article ofclaim 12, wherein the first layer, the second layer, or both, have amean surface porosity from about 10% to about 70%.
 19. The article ofclaim 12, wherein the cross-linking agent is selected from the groupconsisting of boric acid, ethylenediamine, butylenediamine,p-phenylenediamine, glutaraldehyde, epichlorohydrin, phosphorusoxychloride, trimetaphosphate, linear mixed anhydrides of acetic and di-or tribasic carboxlic acids, vinyl sulfone, diepoxides, cyanuricchloride, aldehydes, formaldehyde, acetaldehyde, acrolein,1,4-butanediol diglycidyl ether, glyoxal, glyoxylic acid, oxydisuccinicacid, citric acid, fumaric acid, and combinations thereof.
 20. Thearticle of claim 12, wherein the particles are selected from the groupconsisting of cellulose nanofibers, polyhedral oligomericsilsesquioxane, silica nanoparticles, carbon nanotubes, carbonnanofibers, and combinations thereof.
 21. The article of claim 12,wherein the spacing between the first layer and the second layer is fromabout 0.3 nm to about 100 nm.